This invention relates to catalysts and catalytic processing of sulfur dioxide to elemental sulfur.
A chronic concern for the environment has been release of pollution from industrial and other sources into the air and water. In order to avoid some of the local effects of air-born containments from manufacturing processes, these materials were released from the top of lengthy smokestacks. However, such smokestack industry emissions still added to the general environment contamination.
Of particular concern to environmentalists are gaseous emissions containing sulfur dioxide. When this gas rises to cloud level, the rain produced from these clouds can become highly acidic, reaching the acidic levels of vinegar. Because the emissions and clouds effected by them can travel great distances beyond the point of the initial emission, this form of pollution takes on international dimensions.
The effects of such industrially related acid rains are infamous. Streams and lakes in North West America and Canada have been rendered devoid of their natural flora and fauna due to acidification by acid rain. Trees in these areas have also been badly compromised. Similar effects have been seen in Europe, where large sections of trees in the famous Black Forest have been damaged, and in some cases, destroyed by the effects of acid rain.
In order to minimize the release of sulfur dioxide into the atmosphere, smokestack "scrubbers" have been developed. These are devices which to some degree remove sulfur dioxide from flue gases emitted by such facilities as power plants. Most of the presently available processes capture sulfur dioxide, and then convert this gas to a waste material. Examples of such waste materials are calcium sulfite and calcium sulfate.
The disadvantage of producing secondary waste materials from sulfur dioxide is that they, in turn, require disposal, and are ultimately released into the environment. Large amounts of sulfur dioxide are produced by activities requiring continuing combustion processes, such as power plants. As a result, the solid wastes produced by standard sulfur dioxide conversion methods represent an environmental problem in their own right.
In response to this problem with current SO.sub.2 capture methods, researchers are attempting to develop regenerable flue gas desulfurization means and process. Some of these attempts to limit or avoid the production of contaminating solids have been commercialized, while others have not progressed beyond basic research.
In regenerable processing of sulfur dioxide, this gas must first be captured in some form. In the Welman-Lord, Tung, Cansolv, and Dow processes, sulfur dioxide from flue gas is first absorbed into an alkaline solution. In the NOXSO and CuO processes, the sulfur dioxide is adsorbed on a solid substrate and subsequently desorbed to produce a stream of high concentration sulfur dioxide.
In would be highly desirable to convert sulfur dioxide recovered from smokestack scrubbers to elemental sulfur. If this conversion could be accomplished in a commercially feasible fashion, it would facilitate storage and transportation of the waste products. It would also allow reclamation of sulfur and its recycling as a valuable chemical.
Preliminary research efforts have been made to allow the conversion of concentrated sulfur dioxide to elemental sulfur. In these methods, sulfur dioxide is reduced with synthesis gas. These gases are derived from coal (H.sub.2 /CO.dbd.0.3-1.0) or methane (H.sub.2 /CO.dbd.3). At elevated temperatures, sulfur dioxide can be converted to elemental sulfur according the following reaction: EQU 0.875 SO.sub.2 +0.75 H.sub.2 +CO&gt;0.4125 S.sub.2 +CO.sub.2 +0.75 H.sub.2 O (1) EQU 2 SO.sub.2 +3 H.sub.2 +CO&gt;S.sub.2 +CO.sub.2 +3 H.sub.2 O (2)
Sulfur dioxide can also be reduced with natural gas (mainly methane) EQU 2 SO.sub.2 +CHO.sub.4 &gt;S.sub.2 +CO.sub.2 2 H.sub.2 O (3)
These reactions must be facilitated with catalysts in order to achieve a real time high conversion efficiency of SO.sub.2. Even with the assistance of numerous catalysts, commercially feasible conversion efficiencies have not been achieved.
In addition to elemental sulfur, the above reactions produces a number of undesirable byproducts. These can include hydrogen sulfide, carbonyl sulfide, carbon disulfide, and elemental carbon. These byproducts complicate the ability of the conversion reactions to effectively reduce the net airborne contaminants produced during industrial processing.
Because of the inadequacies of the above reactions when directed to actual industrial applications, research efforts have been carried out to bring this potentially useful area of technology to a level where it has practical applications. The thrust of these research efforts have been to improve the conversion efficiency of sulfur dioxide and increase the selectivity to the production of elemental sulfur at relatively low temperatures.
While there has been some success in this area of research, the results which have been reported to date can not practically be applied to commercial uses. Akhmedov, et al developed catalysts to facilitate the above reactions. These researchers were able to achieve a number of promising sulfur yields using a variety of catalysts. Using a bauxite-bentonite catalyst, a 64-65% sulfur yield was obtained at 350.degree. C. with a feed gas at a molar ratio (CO+H.sub.2)/SO.sub.2 of 2 and a space velocity of 1000 h.sup.-1 (Akhmedov et. al., Azerb. Khim. Zhi., Vol. 2, p. 95, 1983). A NiO/Al.sub.2 O.sub.3 catalyst produced a 82.0% sulfur yield at 300.degree. C. with a space velocity of 5000 h.sup.-1. (Akhmedov et. al. Zh. Prikl. Khim., Vol. 1, p. 16, 1988.) Some of this group achieved a 82.0% and 87.4% sulfur yield with a Co.sub.2 O.sub.3 /Al.sub.2 O.sub.3 catalyst at 300.degree. C. with a space velocity of 1000 h.sup.-1 ; and 500 h.sup.-1 respectively. (Akhmedov et. al, Zh. Prikl. Khim., Vol. 8, p. 1891, 1988.) They also found a 82.3% and 78.6% sulfur yield with a NiO+Co.sub.3 O.sub.4 catalyst at 400.degree. C. with a space velocity of 500 h.sup.-1 and 1000 h.sup.-1 respectively. (Akhmedov et. al., Khim. Prom., Vol. 1, p. 37, 1989.)
While the prior research provides tantalizing possibilities for the practical conversion of sulfur dioxide to elemental sulfur, there are a number of severe limitations to the technology as it is presently developed.
For instance, a 90% or better yield of elemental sulfur is important for the practical application of this technology to smokestack industries. As can be seen from the above discussion of the prior art, even pushing other aspects of the processing parameters to the limit, researchers have not been able to achieve yields at a 90% or higher level.
High space velocity is another important factor in making this technology practically available for industrial use. Space velocity is the factor describing the amount of catalyst required to process a given amount of sulfur dioxide within a proscribed time. Prior research efforts have required a very large reactor as compared with the rate of conversion. The space velocity factor alone can keep this technology from having practical applications. For instance, the limitations of the technology as presently developed are not sufficiently cost effective to be applied to standard power plant operation.
For methods for sulfur dioxide conversion to elemental sulfur to be practically applied to smokestack industries, it would be necessary to develop catalysts and methods which have high space velocities and conversion rates which could be achieved at relatively low temperatures.